AAD sync includes the mailboxGUID

Hi, 

A few questions and best practice recommendations. 

I have synced Local AD with Azure using AAD. This works. UPN is soft matched and all ID's is correct. 

We still have an Exchange 2010 locally, and the MailboxGUID is synced with the Azure AD. 

50% of the users have been registered as Contacts in Exchange Online. Their Mail address is correct.

After some research I see the issue is with the MailboxGUID is matched the user online. Removing the MailboxGUID locally is not an option as this will result in issues with the mailbox itself. 

In the rules editor I can "choose" to not include this attribute, but what will the result be for the users existing Online and ?

Is there a way to "test" this with only one user ?

Any recommendations ?